The present invention relates to the field of treatment of gases discharged at the exhaust of lean-burn spark-ignition engines and of diesel engines.
Such engines emit a certain number of pollutants that have to be eliminated, which must be done all the more efficiently as standards are becoming increasingly severe, notably in industrialized countries.
Examples of the most numerous pollutants that have the most harmful effect on the environment are nitrogen oxides.
It is well-known to eliminate this type of pollutant by passing the exhaust gases through catalysts (referred to as DeNOx catalysts) intended for nitrogen oxides conversion. This requires postinjection of reducers such as hydrocarbons for example. The known catalysts being active within a given temperature range, several catalysts having different formulations, i.e. different activity ranges, can be placed in the catalytic muffler. The sphere of action of the catalytic elements is thus extended.
By way of example, the formulations used for low temperatures are of the Platinum/Alumina or Platinum/Zeolite type. The temperatures for which these catalysts are the most active range from 200xc2x0 C. to 250xc2x0 C.
Catalysts referred to as xe2x80x9cHigh-temperaturexe2x80x9d catalysts are generally active between 300xc2x0 C. and 500xc2x0 C. These are for example Copper-Zeolite type catalysts.
However, in this context, a problem arises when the exhaust gases are not, in the catalyst(s), within a temperature range for which conversion of the nitrogen oxides is sufficient.
However, the global efficiency of such aftertreatment systems remains limited. By way of example, the efficiency on nitrogen oxides of a Platinum/Alumina catalyst with postinjection of gas oil is commonly below 50%.
There are also catalysts on which the nitrogen oxides are adsorbed in various forms. By way of example, the NOx can be stored as nitrates or enter an oxide structure. These catalysts are commonly referred to as xe2x80x9cNOx trapsxe2x80x9d.
The xe2x80x9cnitratexe2x80x9d type NOx trap is a catalyst that allows nitrogen oxides to be stored at the surface thereof, in an oxidizing environment. It generally consists of a precious metal deposited on or close to a basic mass which is generally an oxide or a mixture of alkaline oxides, alkaline-earth oxides or rare earths. In lean mixture (excess oxygen), the NO (NOx=NO+NO2) that is predominantly present in exhaust gases is oxidized by the precious metal, thus forming NO2. This NO2 migrates to the catalyst surface where it is adsorbed on the oxide and forms a nitrate. These nitrates are stable in an oxidizing environment over a very wide temperature range. In order to desorb these nitrates from the surface of the catalyst, a high temperature is required in the oxidizing environment or a reducing mixture has to be made.
Since exclusively thermal regeneration of the NOx trap does not allow treatment of the nitrogen oxides redischarged at the exhaust, a second catalyst (continuous DeNOx type for example) is then necessary to reduce them.
Patent application EP-A1-0,540,280 describes thermal regeneration of this type, with a NOx trap, provided with a gas heating system, followed by a nitrogen oxides reduction catalyst. Both catalysts are mounted on a line bypassing the main exhaust line. According to this document, a valve system allows to decrease the GHSV (ratio of the flow of gas to the volume of catalyst expressing the contact time between the gases and the catalyst) during trap emptying stages. The NOx conversion coefficient of the nitrogen oxides reduction catalyst is thus improved. However, with this configuration, the part of the gas stream flowing through the main line does not flow through the NOx reduction catalyst.
Regeneration through gas mixture strength control allows to reduce the desorbed NOx by three-way type catalysis, by depositing a suitable noble metal on the catalyst (rhodium for example).
With a gasoline engine running with a lean mixture, transition from lean to rich is compatible with its function mode; on the other hand, with a diesel engine, it is more difficult to obtain a mixture strength above 1.
A known implementation consists in injecting hydrocarbons into the exhaust line upstream from the catalyst when the nitrates have to be desorbed from the NOx trap. U.S. Pat No. 5,201,802 illustrates an embodiment of this type. However, although this method allows to obtain mixture strengths momentarily above 1, the gaseous mixture obtained contains high concentrations of oxygen, which is disadvantageous for regeneration.
Another known process consists in reinjecting exhaust gases at the intake at very high rates and in controlling the mixture strength at the engine intake. Document EP-A1-0,829,623 discloses such a process.
The latter strategy has the drawback of disrupting the smooth running of the engine and of making engine control more complex.
The present invention comprises regeneration of a NOx trap essentially based on variation of the mixture strength of the exhaust gases, and which does not disrupt the smooth running of the engine.
Regeneration of a NOx trap includes here both NOx emptying and reduction, the latter being carried out by the NOx trap.
More precisely, the object is to allow regeneration of the NOx trap by decreasing the oxygen concentration and by increasing the carbon monoxide CO and hydrogen H2 concentrations in the exhaust gases upstream from the NOx trap. CO and H2 are, by definition, good reducers as they result from partial oxidation of postinjected hydrocarbons. The carbon monoxide acts both on NOx emptying and reduction.
The object of the present invention thus is a device for eliminating nitrogen oxides in an exhaust line of a lean-burn internal-combustion engine, comprising a means for trapping the nitrogen oxides, a means for regenerating said nitrogen oxides when the trapping means is saturated, a hydrocarbon treating means arranged upstream from the nitrogen oxides trapping means, a hydrocarbon injection means placed upstream from the hydrocarbon treating means, a means for measuring the gas mixture strength.
According to the invention, the hydrocarbon treating means is a partial (or controlled) hydrocarbon oxidation catalyst that cooperates with said nitrogen oxides trapping means that allows to obtain, at the outlet thereof, gases with a low oxygen (O2) concentration and with high carbon monoxide (CO) and hydrogen (H2) concentrations. The device according to the invention further comprises a means for recording and for processing data received from the various detectors and/or stored so as to allow effective regeneration of the NOx trap without disrupting the smooth running of the engine.
According to an embodiment of the invention, the hydrocarbon injection means, the hydrocarbon treating means and the NOx trapping means are arranged in this order and in series in relation to the direction of circulation of the gases in the exhaust line.
According to a particular embodiment of the invention, the hydrocarbon injection means, the hydrocarbon treating means and the NOx trapping means are arranged in the main exhaust line itself.
According to another possibility, the hydrocarbon injection means, the hydrocarbon treating means and the NOx trapping means are arranged in a line bypassing the main exhaust line, the device according to the invention then comprising a means for modulating the flow of the gases between said bypass line and the main line.
Without departing from the scope of the invention, a hydrocarbon injection means, a hydrocarbon treating means and a nitrogen oxides trapping means can thus be arranged both in the bypass line and in the main line, with a means for modulating the flow of the gases between said bypass line and the main line.
Furthermore, the device according to the invention can comprise at least one temperature detector that can be placed upstream from said hydrocarbon treating means.
The device according to the invention can also comprise a means for measuring the amount of NOx trapped in the trapping means, downstream from the latter.
Furthermore, a second temperature detector can be placed in the exhaust line, downstream from the hydrocarbon treating means.
In particular, the means for measuring the exhaust gas mixture strength can be placed downstream from the NOx trapping means.
Without departing from the scope of the invention, the means for measuring the exhaust gas mixture strength can be placed between the hydrocarbon treating means and the NOx trapping means.
Furthermore, the exhaust line can comprise a gas preheating means placed upstream from the hydrocarbon treating means.
The present invention also relates to a process for eliminating nitrogen oxides in an exhaust line of a lean-bum internal-combustion engine, characterized in that it consists in:
trapping the NOx in a suitable means,
injecting hydrocarbons into the exhaust line according to various engine running parameters and to the state of saturation of a NOx trapping means,
partly oxidizing the hydrocarbons in a specific means so as to obtain a maximum amount of CO and H2,
regenerating said NOx trapping means by means of the hydrocarbon oxidation products, notably CO and H2.
According to an aspect of the invention, the amount of NOx stored in the NOx trapping means is monitored.
According to another aspect of the invention, the temperature of the gases upstream and/or downstream from the hydrocarbon treating means is monitored.
Furthermore, the gas mixture strength upstream and/or downstream from the NOx trapping means is monitored.
Said additional hydrocarbons are advantageously injected when the NOx trapping means is saturated, the temperature (T1) of the exhaust gases is above a threshold value (TR) for which the hydrocarbon treating means is active, so that the exhaust gas mixture strength (xcex) is greater than or equal to a given mixture strength (xcexR) that triggers regeneration of the trapping means.
More precisely, said hydrocarbons are injected for a length of time (dR) that is shorter than a predetermined stored time (dRMAX).
According to an advantageous aspect of the invention, the exhaust gas flow is modulated between a main line and a line that bypasses said main line.
The exhaust gases are thus furthermore heated prior to being partly oxidized in said bypass line.
In particular, regeneration is stopped according to the information provided by a detector such as a mixture strength probe placed downstream from the NOx trapping means.